<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 500px;
            height: 500px;
            border: 2px solid black;
            margin: 100px auto;
            display: flex;
        }
        .box div{
            width: 100px;
            height: 100px;
            border: 1px dashed red;
        }
        .div1,.div3{
            flex: 1;
        }
        .div2{
            flex: 2;
            /* 占满剩余空间 */
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="div1"></div>
        <div class="div2"></div>
        <div class="div3"></div>
    </div>
</body>
</html>